1. Field of the Invention
The present invention relates generally to a miniature piezoelectric motor, and in particular to a piezoelectric ultrasonic motor for 2-dimensional positioning.
2. Description of the Related Art
Piezoelectric ultrasonic motors with their exceptional properties, such as fast response, high precision, absence of parasitic magnetic fields, frictional locking at the power-off stage, high power-to-weight ratio, and smaller package size as compared to conventional electromagnetic motors, are readily adaptable for use in a variety of electronic devices. These devices range from compact, consumer electronic devices, such as cellular phones, digital cameras, computers, video recorders, to optoelectronic devices used in optical communications. The popularity of these types of electronic devices continues to influence the development of low cost, simple, and more compact piezoelectric ultrasonic motors.
The characteristics of a piezoelectric motor, including a faster response time, a high power-to-weight ratio, no extrinsic or intrinsic magnetic fields and smaller packaging capability, male them advantageous over a conventional electromagnetic motor. However, in the past their use has been compromised by other characteristics, including the need for high voltage, high frequency power sources, and potential wear at the rotor/stator interface.
The piezoelectric motor operates using a ferroelectric ceramic element to excite ultrasonic vibrations in a stator structure. The elliptical movement of the stator is converted into the motion of a sliding plate in frictional contact with the stator. The resulting movement is either rotational or linear, depending on the design of the structure. A conventional piezoelectric ultrasonic motor can generally be classified into two classes: 1) elliptical motion at the motor tip ultrasonic motors, and 2) standing wave ultrasonic motors. The disc-type or ring-type elliptical motion at the motor tip (rotary motor) may be fabricated from a piezoelectric disc (or ring) and a metal disc. The operation of the linear piezoelectric motor is based on the excitation of a longitudinal and a superimposed bending mode of a rectangular piezoelectric plate, to achieve the elliptic motion at the driving tip of the motor.
Previous linear ultrasonic motors were driven by the elliptic motion of the combined displacement of longitudinal (d31) and secondary bending modes. These motors generally operated according to the principle, that at a certain distance-to-length ratio of a rectangular-shaped piezoelectric ceramic plate, the resonant frequencies of first longitudinal and second bending modes coincide with each other. The elliptic motion at the motor tip is obtained by the combination of the two vibrations. Another example of a linear piezoelectric motor uses vibrators at right angles. However, this motor exhibited higher thrusts and speed than other motors.
While these previously described piezoelectric ultrasonic motors work in products where a small displacement with high accuracy is desirable, their use is restricted by the associated manufacturing costs. Thus, there is a need in the art for a simple, low cost, compact piezoelectric linear ultrasonic motor for 2-dimensional motion.